The operation of spin torque devices is based on the phenomenon of spin transfer torque. If a current is passed through a magnetization layer, called the fixed layer, it will come out spin polarized. With the passing of each electron, its spin (which is angular momentum of the electron) will be added to the magnetization in a next magnetic layer, called the free layer, and will cause a small change in the free layer. This is, in effect, a torque-causing precession of magnetization in the free layer. Due to reflection of electrons, a torque is also exerted on the magnetization of an associated fixed layer. In the end, if the current exceeds a certain critical value (given by damping caused by the magnetic material and its environment), the magnetization of the free layer will be switched by a pulse of current, typically in about 1 nanosecond. Magnetization of the fixed layer may remain unchanged since an associated current is below its critical value due to geometry or due to and adjacent antiferromagnetic layer.